I dunno, when I think of macro, I think 'beyond normal human scale' 9 feet seems like a good number for macro, but if we're talking about GIANT macro, 20 feet seems more fitting. But in terms of personal prefrence, I like 9 feet.

Beoweasel, mighty ferret Hero on a quest of great importance, to find coin and cleavage...

I generally consider (though there can be situational deviations) that the term 'macro' involves a ratio of at least 2:1. So, with that criterion, and a 'normal' six-foot human, twelve feet in height would be the beginning of the macro realm in that situation.

I consider anyone who can't stand up in a regular-height, single-story room without their head bursting through the ceiling to be macro. If I'm not mistaken, most ceilings are at about 12', so anything above that--20 being a good round number, to be certain--would be considered macro. Below 12', and they're just...hot.